Recently, as existing energy resources like oil and coal and the like are expected to be exhausted, much attention is increasingly paid to alternative energy sources which can be used in place of the existing energy sources. As an alternative energy sources, sunlight energy is abundant and has no environmental pollution. Therefore, more and more attention is paid to the sunlight energy.
A photovoltaic module converting sunlight energy into electrical energy has a junction structure of a p-type semiconductor and an n-type semiconductor. When light is incident on the photovoltaic module, an electron with a negative electric charge and a hole with a positive electric charge are generated by interaction between the light and a material constituting the semiconductor of the photovoltaic module. Then, electric current flows while the electron and the hole move.
Depending on the thickness of the semiconductor of the photovoltaic module, the photovoltaic module is classified into a bulk type photovoltaic module and a thin-film type photovoltaic module. The thin-film type photovoltaic module includes a photovoltaic material layer of which the thickness is equal to or less than from several tens of micrometers to several micrometers.
At present, a bulk type silicon photovoltaic module is mainly and widely used for ground power. However, the recent increase of the demand for the bulk type silicon photovoltaic module is now increasing its price due to the lack of its raw material.
Therefore, in recent times, providing an integrated thin-film photovoltaic module has become the most important issue, which has high energy conversion efficiency and can be mass produced at a low cost. However, a single-junction thin-film photovoltaic module is limited in its achievable performance. Accordingly, a double junction thin-film photovoltaic module or a triple junction thin-film photovoltaic module having a plurality of stacked unit cells has been developed, pursuing high stabilized efficiency. The double junction thin-film photovoltaic module and the triple junction thin-film photovoltaic module are called a tandem type photovoltaic module.
In addition to this, researches are now being devoted to an integration technology for the photovoltaic module in order to improve the efficiency of the thin-film photovoltaic module by reducing the ineffective area thereof.